The term “high molecular weight polyethylene” is generally used to define polyethylene having a molecular weight of at least 3×105 g/mol as determined by ASTM 4020 and, as used herein is intended to include very-high molecular weight polyethylene or VHMWPE (generally characterized as polyethylene having a molecular weight of at least 1×106 g/mol and less 3×106 g/mol as determined by ASTM 4020) and ultra-high molecular weight polyethylene or UHMWPE (generally characterized as polyethylene having a molecular weight of at least 3×106 g/mol as determined by ASTM 4020). High molecular weight polyethylenes are valuable engineering plastics, with a unique combination of abrasion resistance, surface lubricity, chemical resistance and impact strength. Thus, in solid, compression molded form, these materials find application in, for example, machine parts, linings, fenders, and orthopedic implants. In sintered porous form, they find application in, for example, filters, aerators and pen nibs.
Currently, high molecular weight polyethylenes are generally produced using Ziegler-Natta catalysts, see, for example, EP186995, DE3833445, EP575840 and U.S. Pat. No. 6,559,249. However, these catalysts have certain limitations with regard to the molecular weight and molecular weight distribution of the polymers that can be produced. There is therefore significant interest in developing alternative catalyst systems for producing high molecular weight polyethylene.
Other known catalysts for olefin polymerization are single site catalysts. According to the present state of technology, high molecular weight polyethylenes are manufactured using these catalysts only in exceptional cases and under economically unprofitable conditions. Thus, with heterogeneous constrained-geometry catalysts, high molecular weight polyethylene is produced only with moderate activity and increased long chain branching, which can lead to reduced hardness and abrasion properties. With so-called phenoxy-imine catalysts, high molecular weight polyethylene is obtained only at low activity at economically disadvantageous temperature levels. Examples of these and other metallocene catalysts are described in WO9719959, WO0155231, Adv. Synth. Catal 2002, 344, 477-493, EP0798306 and EP0643078.
One other potentially useful catalyst system for producing high molecular weight polyethylene comprises a Group 4 metal complex of a bis(phenolate) ether deposited on a particulate support, such as silica. Such a catalyst system is disclosed in International Publications Nos. WO 2003/091262 and WO 2005/108406, the entire disclosures of which are incorporated herein by reference. Research has, however, shown that, although this system provides an effective catalyst for the slurry phase polymerization of polyethylene with molecular weights unachievable with Ziegler-Natta catalysts, control of the polymerization process is unexpectedly dependent on the presence of a narrowly defined amount of an antistatic agent in the polymerization medium.
U.S. Pat. No. 7,157,532 discloses a process for preparing olefin homopolymers or copolymers by polymerization of at least one olefin in a hydrocarbon (mixture) in the presence of a molar mass regulator, a mixed catalyst and a substance which increases the electrical conductivity of the hydrocarbon (mixture) and is soluble in the hydrocarbon (mixture) or which reacts with components of the mixed catalyst. The addition of the substance which increases the electrical conductivity of the hydrocarbon is said to reduce the tendency of the polymer particles to form agglomerates and deposit on the reactor walls, without the activity of the catalyst system being adversely effected. However, the mixed catalyst employed in the '532 patent is a Ziegler-Natta type catalyst obtained by reaction of a magnesium alkoxide with titanium(IV) halide and an organic aluminum compound.
U.S. Pat. No. 7,205,363 discloses a polymerization process comprising contacting: (a) a catalyst system, particularly a supported metallocene catalyst system; (b) monomers comprising at least 85 wt % propylene monomers by total weight of the monomers; and (c) an antistatic agent that has been pre-contacted with a scavenger; in a reactor under polymerization conditions; wherein the antistatic agent is present from about 0.3 to 1.5 ppm based on the weight of the monomers introduced into the reactor. The antistatic agent can comprise a polysulfone copolymer, a polymeric polyamine, an oil-soluble sulfonic acid, or mixtures thereof, with or without a solvent.